The thin, flexible, revolving endless metal casting belts intended to be employed in machines for the continuous casting of metals are normally made by cutting off a length of wide, thin, strip metal stock and then joining the cut ends by welding the ends together to form an endless casting belt of considerable width. For twin-belt continuous casting, the belts are typically required to be flattened or leveled after welding fabrication and before use, because the weld joining of the ends of the strip stock during fabrication necessitated subsequent leveling of the endless belt. Furthermore, commercial wide, thin metallic strip for use as belt stock as delivered is often not normally flat enough to use in twin-belt continuous casting machines unless the fabricated belts are leveled. This condition of being not normally flat enough is generally true with both ferrous and non-ferrous metallic belt materials.
The usual prior art method of leveling belts involved two simultaneous mechanical influences upon the belt in a process that may be called roller-stretch leveling.
The first such influence was the application of a uniform tensile force to the endless metallic belt. The belt to be leveled was placed around two (or more) pulley rolls mounted on a carriage frame. The requisite longitudinal tensile force within the casting belt to be leveled was induced by outwardly moving a pulley roll against the belt. The pulley roll was moved uniformly outwardly against the inside surface of the endless belt until the pulley roll took up the slack in the belt and forcibly tensed the belt. The tension so induced was usually in the range from one-twentieth to one-third of the yield stress of the belt material, though the roller-stretch leveling process will sometimes work suitably outside of this tension versus yield stress range.
This tensile force was not enough by itself to render the belt level. The second mechanical influence was the operation of revolving the tensed belt against and past at least one relatively small diameter, cylindrical, transversely disposed work roller. This small diameter roller deflected the course of the belt in such a way as to cause inelastic yielding elongation of the belt progressively, successively more uniformly across the full belt width as the belt repeatedly contacted and passed the small diameter roller during its revolutions. Revolving of the belt was continued until ultimately this operation stretched all areas of the belt uniformly, as desired. This small diameter work roller was cylindrical; that is, it had the same constant and uniform diameter along its entire working length.
The uniform diameter of this prior work roller was conveniently in the range from about 200 times the belt thickness to about 20 times the belt thickness, with a preferred diameter being about 60 to about 80 times the thickness of the belt being leveled. The belt thickness was typically in the range from about 0.035 to about 0.065 of an inch (about 0.9 to about 1.7 mm), though the thickness could be somewhat outside of this range.
The inelastic yielding elongation which resulted in leveling ultimately occurred essentially uniformly across the full belt width, occurring only during the continuing revolution of the belt and then only at two places along the small diameter work roller. The first inelastic yield place was along the narrow straight zone where the revolving belt first contacted and became wrapped around the work roller. The second inelastic yield place was along the narrow straight zone where the belt last contacted the roller and ceased to be wrapped upon it. The revolving tensed belt, upon entering from a straight tangent path onto the curved surface of a work roller, bent inelastically uniformly across its width into a curve which may conform, in the limit, to the shape of the roller; i.e., the mutual contact between the surface of the tensed belt and the surface of the uniform small diameter work roller produced inelastic yield in bending and elongation which ultimately became uniform across the belt width after continuing revolutions of the belt. Inelastic bending and elongation occurred again, with similar ultimate results, when the belt left the work roller to begin a new straight tangent path, since the tension in the belt forced it to resume a straight course. A second work roller was usually employed, on the opposite side of the belt, near to but not directly opposed to the first one for producing significant deflection or bending of the belt in the opposite direction from the first work roller.
In such prior roller-stretch leveling, no "rolling" of the belt material between two directly opposed pressure rolls was involved; that is, no pressure was applied whereby the belt would be squeezed in between two directly opposed rollers. Indeed, the uniform small diameter work rollers were advantageously rubber covered, in order to avoid inadvertent causing of dimples from tiny bits of debris which might adhere to the work roller and to avoid undesirable bending down of tiny asperities raised by the grit-blasting process that was performed on the outside surface of many belts prior to such uniform effect leveling. Such grit-blasting is described in U.S. Pat. Nos. 4,487,157; 4,487,790 and 4,588,021. The roller-stretch leveling was carried out subsequent to grit-blasting.
There was a "tailing-off" involved in completion of the uniform effect roller-stretch leveling during which the deflection and bending of the belt was progressively reduced for achieving an esentially uniform final condition around the full circumference of the endless belt. The final contact of the revolving belt with the work rollers should occur under conditions where the bending is minimal, i.e., when a work roller has been retracted far anough from the other roller (or rollers) to result in only slight bending of the belt as it passes by each work roller. Alternatively, the belt tension was slackened gradually during this tailing-off. The overall prior art result was that the belt was rendered both uniformly flat and practically free from residual internal tensile, compression or bending stresses, i.e., the resulting stress condition of the belt was essentially uniform across its full width and over its full endless circumference.
The essence of the prior art roller-stretch belt leveling method and apparatus was disclosed in U.S. Pat. No. 2,904,860 of C. W. Hazelett, notably in column 8, and in FIGS. 1, 2, and 4 therein. The roller-stretch belt leveling apparatus with refinements was incorporated into a number of continuous casting machines that were manufactured and sold to the metals industry by the assignee of the present patent application. Such mechanisms are indicated in U.S. Pat. No. 3,848,658 (FIGS. 1, 2 and 4); U.S. Pat. No. 3,878,883 (FIGS. 1 and 2); U.S. Pat. No. 3,949,805 (FIGS. 1 and 2); U.S. Pat. No. 3,963,068 (FIGS. 1 and 2) and U.S. Pat. No. 4,002,197 (FIG. 1), all referenced herein. In these prior roller-stretch belt-leveling mechanisms, the belt itself was under uniform tension across its width and was also at essentially the same temperature across the full width of the belt as the belt was repeatedly deflected around the cylindrical small diameter work roller during continuing revolution of the belt, so that the resultant inelastic yielding elongation which occurred ultimately became essentially uniform in effect across the full width and length of the revolving belt. The intention of the prior art was to achieve uniformity of a stress-free condition across the full belt width and along the full belt circumference.
A leveling mechanism can be mounted upon a carriage of a continuous casting machine, as illustrated in the above-listed patents. Also, separate machines for leveling of belts have been built which operate on the same principles, utilizing two or more pulley rolls around which the belt was revolved during roller-stretch leveling for achieving an essentially uniform effect across the belt width and along the belt circumference.
Whether performed on the casting machine or elsewhere, the uniform leveling of wide belts in the prior art presented the problem that the long thin uniform diameter work rollers of the desired small diameter were not rigid enough in the bending mode over their length. They would bend elastically and so spoil the desired uniformity of bending and leveling across the width of the belt. The solution was to "back up" the work rollers, i.e. to rigidly support these small diameter work rollers along their full length to prevent bending, by means of firmly and accurately positioned, rigidly mounted rotating support elements, either continuous or placed at closely spaced intervals, thereby keeping the axis of the work roller straight. Over many years, the present assignee has delivered casting machines to the metals industry that incorporated roller-stretch belt leveler apparatus based on these principles with back-up, rotating support elements for preventing bending of the small-diameter work rollers, for keeping the axis of the work roller straight.
In the prior art, the most desirable condition of belts wa presumed and intended to be that of uniform freedom from internal residual stresses, in order to allow the belts to present in the mold a flat surface to the metal product being frozen. Accordingly, the belt leveling equipment of the prior art was designed to achieve that intended uniform result across the full width of the belt. The work roller or rollers were cylindrical in shape,--i.e. of the same constant and uniform diameter throughout the entire working region of the smooth periphery of the work roller and the belt was under uniform tension across its width and also was at essentially the same uniform temperature across the width of the belt as the belt was bent around the work roller for achieving uniformity of stress-free residual effect across the full width and along the full length of the belt.